1. Field of the Invention
The present invention relates to a driving force distribution device that distributes driving force between right and left wheels of front wheels or rear wheels in a four-wheel drive vehicle, and particularly, the present invention relates to a driving force distribution device capable of canceling a limitation on driving force to be transmitted to right and left wheels on the basis of a difference between wheel speeds of right and left wheels.
2. Description of the Related Art
Heretofore, in order to distribute driving force transmitted from an engine (driving source) via a propeller shaft to right and left rear wheels, a four-wheel drive vehicle provided with a driving force distribution (torque distribution) device has been known (for example, see Japanese Patent No. 3406169, hereinafter, referred to as “Patent Literature 1”). In such a driving force distribution device, two clutches are respectively provided on the right and left of a hypoid gear or bevel gear coupled to a rear end portion of the propeller shaft. Such a driving force distribution device (or driving force control device) is provided within a rear differential of the four-wheel drive vehicle.
In this driving force control device for the four-wheel drive vehicle disclosed in Patent Literature 1, in order to make gears and/or clutches in the rear differential smaller, such control that the maximal transmitted torque for any clutch is reduced as vehicle speed is increased is carried out.
However, in a conventional control method as described in Patent Literature 1, the maximal transmitted torque to be transmitted to each of right and left rear wheels is first reduced, and engagement force of each of the right and left clutches of the rear differential is then increased or reduced within a range in which the engagement force does not exceed the maximal transmitted torque, by which a yaw motion of the four-wheel drive vehicle is controlled. For that reason, the maximal transmitted torque is to be reduced as the vehicle speed becomes faster. Thus, there has been a problem that it is hard to exert an effect obtained by controlling the yaw motion in such a situation.
Further, in the conventional driving force distribution device, in the case where an upper limit of the maximal transmitted torque for each of the right and left wheels is set to 1,200 Nm, for example, a hypoid gear portion to which the total torque of the rear wheels is transmitted has been designed to have a torque capacity (total torque capacity) of 2,400 Nm, for example, which is a sum of the upper limits of the maximal transmitted torque of both of the right and left wheels. However, the maximal driving torque of each of the right and left wheels required at straight ahead driving is different from the maximal driving torque of each of the right and left wheels required at turning driving. Therefore, the maximal torque actually needs not to be outputted to both of the right and left wheels at the same time, and the hypoid gear portion is allowed to reduce the torque capacity to the total torque capacity or less, for example, 1,600 Nm. It is desired to make the hypoid gear portion further smaller by designing it in this manner.